1. Field of the Invention
The present invention relates generally to a method of forming interconnection patterns and, more particularly, to an improved method of forming interconnection patterns which enables prevention of corrosion of metal interconnections.
2. Description of the Background Art
In a process of manufacturing a semiconductor device, the step of forming interconnection patterns is an indispensable processing step. FIGS. 4A and 4B are cross-sectional views showing the interconnection pattern forming step.
Referring to FIG. 4A, a gate 31 and an interlayer insulation film 32 are formed on a semiconductor substrate 30. A contact hole 33b for exposing source/drain regions (not shown) of a transistor and a contact hole 33a for exposing a portion of a surface of gate 31 are formed in interlayer insulation film 32. A barrier metal 40 such as TiN or the like is formed on interlayer insulation film 32 so as to coat the side wall and the bottom of contact holes 33a and 33b. An interconnection layer 34 made of an aluminum alloy film is formed on barrier metal 40 to fill contact holes 33a and 33b.
The reasons for formation of barrier metal 40 are as follows. That is, Si of 1-5% is in general mixed in interconnection layer 34 of the aluminum alloy film. Accordingly, if no barrier metal 40 is formed, an Al atom transfers from interconnection layer 34 to semiconductor substrate 30 (silicon substrate), and a Si atom transfers from semiconductor substrate 30 into interconnection layer 34. The transfer of the atoms causes defects in interconnection layer 34. Thus, barrier metal 40 is necessary.
A resist 35 patterned in a predetermined form is formed on interconnection layer 34.
An interconnection pattern 36 is formed by selectively etching interconnection layer 34 with resist 35 used as a mask, with reference to FIGS. 4A and 4B.
As a conventional method of forming interconnection patterns of aluminum alloy, such a method has been provided that interconnection layer 34 of aluminum alloy is subjected to a wet etching process with a mixed solution of phosphoric acid, nitric acid, etc. by employing resist 35. This method, however, has difficulties in formation of a micropattern of 3 .mu.m or less due to an enhanced etching extending beneath resist pattern 35, a so-called under-cutting. Thus, a method employing a reactive ion etching (hereinafter referred to as RIE etching) using a gas of chlorine or a compound containing chlorine, e.g., Cl.sub.2, SiCl.sub.4, BCl.sub.3, etc. has been adopted for formation of such a micropattern.
A description will be given in detail on the conventional method of forming an interconnection pattern by the RIE etching process and further on disadvantages of the method.
Referring to FIG. 5A, a lower insulator film 2 is formed on a semiconductor substrate 1. A barrier metal 3 such as TiN is formed on lower insulator film 2. A metal interconnection layer 4 such as of AlSi, AlSiCu, Mg and so on is then formed on barrier metal 3. A resist pattern 5 of a predetermined form is formed on metal interconnection layer 4.
With reference to FIGS. 5A and 5B, metal interconnection layer 4 and barrier metal 3 undergo the reactive ion etching by employing a halogen-type gas including chlorine such as Cl.sub.2, SiCl.sub.4, BCl.sub.3 and so on, with resist pattern 5 used as mask. This reactive ion etching causes metal interconnection layer 4 and barrier metal 3 to be selectively etched, resulting in formation of an interconnection pattern 4a. At this time, a protection film 6 including halogen is formed on the side walls of resist pattern 5 and interconnection pattern 4a. This protection film 6 serves to suppress isotropic etching and enhance anisotropy. Protection film 6 containing halogen is made by a complicated reaction of components in the resist, interconnection layer and halogen-type gas.
Then, resist 5 is removed by etching, to complete the formation of the interconnection pattern. However, when this semiconductor device is taken out in the atmosphere with protection film 6 adhering thereto, there occurs a problem that interconnection pattern 4a becomes corroded and then disconnected.
The causes of the corrosion and disconnection of interconnection pattern 4a are as follows. That is, protection film 6 includes halogen, e.g., chlorine, and hence when exposed in the atmosphere, the film reacts with water molecules in the atmosphere and generates hydrochloric acid. This hydrochloric acid acts on interconnection pattern 4a to produce a reaction product (e.g., a metal chloride). When this reaction product becomes separated from the side wall of interconnection pattern 4a, interconnection pattern 4a becomes gradually corroded.
In order to prevent the corrosion of the interconnection pattern, such a processing method has conventionally been proposed that Cl is substituted for F by a plasma processing employing a plasma of a fluorine-type gas such as of CF.sub.4, CHF.sub.3 and so on, with reference to FIG. 5C.
However, the above-described plasma processing employing the gas of CF.sub.4 or the like causes the side wall of barrier metal 3 such as TiN to be etched (hereinafter referred to as side-etch), as shown in FIG. 5C. Thus, it has been impossible to carry out the plasma processing in a sufficient time. That is, sufficient anti-corrosion processing of interconnection pattern 4a has not been accomplished in this method, leading to disconnections of interconnection pattern 4a and to a degradation in reliability of interconnections.